The efficient functioning of the hip joints is important to the well-being and mobility of the human body. Each hip joint includes the upper portion of the femur, which terminates in an offset bony neck surmounted by a ball-headed portion known as the femoral head. The femoral head rotates within a socket, known as the acetabulum, in the pelvis to complete the hip joint. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining of the acetabulum so that the ball of the femur and the hip bone rub together, causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone becoming deformed. This misshapen joint may cause pain and may eventually cease to function altogether.
Operations to replace the hip joint with an artificial implant are well-known and widely practiced. Generally, the hip prosthesis will be formed of two components, namely: an acetabular, or socket, component which lines the acetabulum, and a femoral, or stem, component which includes a weight-bearing ball and replaces the femoral head. During the surgical procedure for implanting the hip prosthesis, the remaining cartilage or damaged tissue is removed from the acetabulum using a reamer such that the native acetabulum will accommodate the outer surface of the acetabular component of the hip prosthesis. The acetabular cup component of the prosthesis can then be inserted into the prepared acetabulum. In some arrangements, the acetabular cup component may simply be held in place by a tight fit with the bone. However, in other arrangements, additional fixing means such as screws and/or bone cement may be used. The use of additional fixing means helps to provide stability in the early stages after the prosthesis has been inserted. In some modern prosthesis, the acetabular cup component may be coated on its external surface with a bone growth promoting substance which will encourage bone ingrowth which helps to hold the acetabular component in place. The bone femoral head also is removed during the surgical procedure, and the femur shaft hollowed out using reamers and rasps to accept the femoral component of the prosthesis. The stem portion of the prosthesis is inserted into the femur and secured therein to complete the hip joint replacement.
In order to strive toward desired performance of the combined acetabular and femoral hip prosthesis components, the acetabular cup portion must be properly positioned in the acetabulum. This is particularly important since incorrect positioning of the acetabular cup component can lead to the prosthetic hip joint suffering from dislocations, a decreased range of motion, and possibly eventual loosening and/or failure of one or both components of the joint.
It is generally believed that there is a preferred orientation for the acetabular cup prosthesis component to provide a full range of motion and to minimize the risk of dislocation. Some example orientations of the acetabular cup prosthesis relative to the acetabular face are 45° to 50° from the vertical and rotated forward to 15° to 20° of anteversion. This broadly replicates the natural angle of the acetabulum. However, the specific angular orientation of the acetabular cup portion varies from patient to patient.
In hip replacement surgery, the acetabular cup portion of the prosthesis is usually oriented in the acetabulum by using an acetabulum positioning instrument. One example of such a positioner is a horizontal arm that is aligned parallel to a predetermined native tissue of the patient when the acetabular cup portion is oriented at a preferred abduction angle. This positioner is therefore sensitive to the position of the patient on the operating table for accuracy. The acetabular cup placement is typically done using an acetabular cup positioner and visual adjustment of the acetabular cup portion to ensure that the horizontal arm of the positioner is approximately parallel to the selected reference tissue (or axis) of the patient. The user of the positioner may also view the position of the acetabular cup portion relative to a second arm on the acetabular cup positioner which is positioned at a preset angle, to assist with positioning the acetabular cup at the correct abduction angle.
However, despite this known positioning procedure, the orientation of the acetabular cup portion in the replaced hip can deviate from the desired orientation. This may be due to one or more factors. First, the positioning of the acetabular cup is usually judged by eye. As the position to be judged by the user is a compound angle, it may be particularly difficult to visualize. Second, since the natural face of the acetabulum is not uniform and—where the hip is arthritic—may be distorted by osteophytes, the acetabulum is not generally a reliable guide for orientating the acetabular cup portion of the prosthetic joint. A third problem is that the prior art mechanical alignment guides usually rely on the pelvis being in a set position which may itself be difficult to judge, particularly in an obese patient. In view of these difficulties, the acetabular cup portion may sometimes be actually located via surgery as much as 20° from the desired/planned position.
The above factors and issues encountered in surgical hip intervention have analogues in the shoulder surgery arena. For example, generally the normal glenoid retroversion of a given patient may fall within a range of approximately 20° (5° of anteversion and 15° of retroversion). (The version of the glenoid is defined as the angle between the plane of the glenoid fossa and the plane of the scapula body.) In the pathologic state, glenoid bone loss may result in a much larger range of version angles.
One goal of shoulder surgery may be to modify the pathologic bone to correct pathologic version to be within the normal range or the normal version of the patient's native anatomy before the bone loss occurred. During surgery, and particularly minimally invasive procedures, the plane of the scapula may be difficult or impossible to determine by direct visual inspection, resulting in the need for assistive devices or methods to define both the pathologic version present at the time of surgery and the intended correction angle.
It is generally believed that there is a preferred orientation for the glenoid component to provide a full range of motion and to minimize the risk of dislocation. Some example orientations of the glenoid prosthesis relative to the glenoid face are about 5° of anteversion to about 15° of retroversion; average version is about 1-2° of retroversion. This broadly replicates the natural angle of the glenoid. However, the specific angular orientation of the glenoid portion varies from patient to patient.
With a view to overcoming these disadvantages, some arrangements have been recently suggested in which a three-dimensional intraoperative computer imaging surgical navigation system is used to render a model of the patient's bone structure. This model is displayed on a computer screen and the user is provided with intraoperative three-dimensional information as to the desired positioning of the instruments and the glenoid component of the prosthetic implant. However, surgical navigation arrangements of this type are not wholly satisfactory since they generally use only a low number of measured landmark points to register the patient's anatomy and to specify the angle of the prosthetic implant component (e.g., a glenoid component), which may not provide the desired level of accuracy. Further, the information provided by such systems may be difficult to interpret and may even provide the user with a false sense of security. Moreover, these systems are generally expensive to install and operate and also have high user training costs. Various proposals for trial prosthetic joint components have been made in an attempt to overcome the problems associated with accurately locating the acetabular cup portion of the prosthetic implant. While these trial systems may help with checking whether the selected position is correct, they are not well-suited to specify the correct position initially, and thus there still is user desire for a system which may assist a user in placement of prosthetic implant component in a prepared native tissue site.